MOS Technology 8563

Lore & Background

The 8563 VDC was originally intended for the Commodore 900, a planned UNIX-based business computer using the Zilog Z8000, but only the Commodore 128 saw production. Unlike earlier MOS video chips like the VIC-II, the VDC had dedicated video memory (16 KB in the original C128, upgradable to 64 KB; 64 KB in the C128DCR) not directly accessible by the microprocessor. The 8563 was difficult to produce, with low initial yields, reliability problems, overheating issues, and timing problems that caused indirect load/store operations on its registers to malfunction. Officially a text-only chip, the VDC's high-resolution bitmap mode was hinted at in technical literature but not described in detail. BASIC 7.0 only supported high-resolution graphics in 40-column mode via the VIC-II. The bitmap mode was later detailed in the Data Becker book 'Commodore 128 - Das große GRAFIK-Buch' (published late 1985 in the US by Abacus Software), with an assembly program by Klaus Löffelmann and Dieter Vüllers. In February 1986, RUN magazine published 'Ultra Hi-Res Graphics' by Lou Wallace and David Darus, which extended BASIC 7.0 for 640×200 graphics; this became the commercial package BASIC 8. Commodore eventually provided complete documentation in the Commodore 128 Programmer's Reference Guide. The VDC lacked sprites but had blitting capabilities for autonomous block memory copies within its video RAM.

Reader's Guide

The 8563 VDC's significance lies in enabling 80-column text and high-resolution graphics on the Commodore 128, distinguishing it from the Commodore 64. Its dedicated video memory and blitting capabilities allowed for rapid screen scrolling and clearing, used by the C128's screen editor ROM. The discovery and documentation of its bitmap mode by third-party authors expanded its utility, leading to utilities like BASIC 8 and use in the GEOS operating system. However, its cumbersome indirect register programming limited frame rates, making it unsuitable for arcade-style games. The VDC's RGBI output was compatible with IBM's CGA standard, and its adjustable character height and attribute system (blink, underline, invert, alternate character set) offered flexibility. The later 8568 DVDC controller, used in C128 DCR models, was more advanced. Overall, the 8563 provided a bridge between home computer and business-oriented display capabilities.

Did You Know?

• The 8563 VDC was originally designed for the unreleased Commodore 900 UNIX computer.
• Early 8563 units had reliability problems and tended to self-destruct from overheating.
• The VDC's bitmap mode was first publicly detailed in a 1985 German book and a February 1986 RUN magazine article.
• The VDC could use up to 512 characters, with adjustable character height taking 16 bytes per character.